Hey Archimus!
I don’t know if your problem is the same or just a similar one, but other players have opened topics about it. Check those as well, I told them about the “fix” I had for when it happened to me, try it please, might help you, too. If not, I’d suggest you also open a ticket because maybe you also had different circumstances for which the error occured for you. 
Check the post " Failure to claim reward from event dungeon" posted by Kellfour. I hope those answers help you as well!